cmd/go/internal/modload: refactor version filtering for exclude

Query and other functions now accept an "allowed" function that
returns an error (previously, the function returned a bool). If the
error is equivalent to ErrDisallowed, it indicates the version is
excluded (or, in a future CL, retracted). This provides predicates a
chance to explain why a version is not allowed.

When a query refers to a specific revision (by version, branch, tag,
or commit name), most callers will not use the Allowed predicate. This
allows commands like 'go list -m' and 'go mod download' to handle
disallowed versions when explicitly requested. 'go get' will reject
excluded versions though.

When a query does not refer to a specific revision (for example,
"latest"), disallowed versions will not be considered.

When an "allowed" predicate returns an error not equivalent to
ErrDisallowed, it may be ignored or returned, depending on the
case. This never happens for excluded versions, but it may happen for
retractions (in a future CL). This indicates a list of retractions
could not be loaded. This frequently happens when offline, and it
shouldn't cause a fatal or warning in most cases.

For #24031

Change-Id: I4df6fb6bd60e3e0259e5b3b4bf71a307b4b32298
Reviewed-on: https://go-review.googlesource.com/c/go/+/228379
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 22 insertions, 6 deletions

diff --git a/src/cmd/go/internal/modload/mvs.go b/src/cmd/go/internal/modload/mvs.go
index 6b6ad945e4..d023ab5094 100644
--- a/src/cmd/go/internal/modload/mvs.go
+++ b/src/cmd/go/internal/modload/mvs.go
@@ -6,6 +6,7 @@ package modload
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -73,16 +74,29 @@ func (*mvsReqs) Upgrade(m module.Version) (module.Version, error) {
 	return m, nil
 }
 
-func versions(path string) ([]string, error) {
+func versions(ctx context.Context, path string, allowed AllowedFunc) ([]string, error) {
 	// Note: modfetch.Lookup and repo.Versions are cached,
 	// so there's no need for us to add extra caching here.
 	var versions []string
 	err := modfetch.TryProxies(func(proxy string) error {
 		repo, err := modfetch.Lookup(proxy, path)
-		if err == nil {
-			versions, err = repo.Versions("")
+		if err != nil {
+			return err
 		}
-		return err
+		allVersions, err := repo.Versions("")
+		if err != nil {
+			return err
+		}
+		allowedVersions := make([]string, 0, len(allVersions))
+		for _, v := range allVersions {
+			if err := allowed(ctx, module.Version{Path: path, Version: v}); err == nil {
+				allowedVersions = append(allowedVersions, v)
+			} else if !errors.Is(err, ErrDisallowed) {
+				return err
+			}
+		}
+		versions = allowedVersions
+		return nil
 	})
 	return versions, err
 }
@@ -90,7 +104,8 @@ func versions(path string) ([]string, error) {
 // Previous returns the tagged version of m.Path immediately prior to
 // m.Version, or version "none" if no prior version is tagged.
 func (*mvsReqs) Previous(m module.Version) (module.Version, error) {
-	list, err := versions(m.Path)
+	// TODO(golang.org/issue/38714): thread tracing context through MVS.
+	list, err := versions(context.TODO(), m.Path, CheckAllowed)
 	if err != nil {
 		return module.Version{}, err
 	}
@@ -105,7 +120,8 @@ func (*mvsReqs) Previous(m module.Version) (module.Version, error) {
 // It is only used by the exclusion processing in the Required method,
 // not called directly by MVS.
 func (*mvsReqs) next(m module.Version) (module.Version, error) {
-	list, err := versions(m.Path)
+	// TODO(golang.org/issue/38714): thread tracing context through MVS.
+	list, err := versions(context.TODO(), m.Path, CheckAllowed)
 	if err != nil {
 		return module.Version{}, err
 	}